1. Field of the Invention
The present invention relates to a structure of a stepping motor and a method of driving the stepping motor, and particularly, relates to a field magnet used in the motor of this type.
2. Description of the Prior Art
Conventional stepping motors of the hybrid type are superior in thrust to the motors of the other types under equal conditions in electric source capacity and motor rating. However, those hybrid stepping motors have a disadvantage in that it is difficult to perform high-speed driving because of the influence of inductance. The disadvantage is described in detail with reference to FIG. 2 which shows a first conventional technique to be improved.
Consideration will be made about the inductance of hybrid stepping motors. In regard to windings 3 and 4, the magnetic circuit of FIG. 2 has a magnetic path in the direction of the arrow. The magnetic reluctance of the circuit is determined mainly by the shape of gaps. The magnetic reluctance R.sub.c as to the windings 3 and 4 is represented by the equation: R.sub.c =R.sub.g1 +R.sub.g2, in which R.sub.g1 and R.sub.g2 respectively represent the magnetic reluctance in the gaps of poles A and A of a stator 1. The inductance of the windings is in proportion to the square of the number of turns N and in reverse proportion to the magnetic reluctance, as represented by the equation: ##EQU1## in which K is a constant.
For driving the hybrid motor at a high speed, it is necessary to make a current for the windings rise rapidly so as to generate large thrust in a short time. Accordingly, the electrical time constant represented by the ratio of inductance to resistance of the windings has a serious meaning.
The electrical time constant of the conventional hybrid stepping motor is of the order of several mS, and various methods have been heretofore used for driving the motor at a high speed with respect to the electrical time constant. In the following, the conventional methods are described in brief.
One of the methods is such that a resistor is connected in series at the outside to increase the value of resistance to thereby minimize the time constant. However, the method has a problem in that the increase of the consumption of electric power by the resistor causes troubles, such as the lowering of efficiency in the stepping motor, the overheat of the motor, and the like.
Another one of the methods is such that the number of turns of the windings is decreased or the gaps are enlarged to increase the magnetic reluctance to thereby reduce the inductance. However, the method has a problem in that the thrust constant of the motor represented by a ratio f/I of the thrust f (newton) to the motor current I (Ampere) is adversely affected and becomes lower under the condition of the same electric source capacity.
As a means for increasing the magnetic reluctance without deterioration of thrust, a method in which a magnet is provided in the magnetic circuit as to the windings, that is, in which a magnet is provided in the magnetic path of the arrow of FIG. 2, is considered. However, in the case where the magnet is merely provided in the path of the arrow, the change of flux interlinkage with the windings according to the change of magnetic reluctance of the gaps is reduced to lower the thrust constant, though the magnetic reluctance is increased corresponding to the magnetic reluctance of the magnet. Accordingly, the reduction in thrust can not be avoided even in this method.
To solve the aforementioned problems, a motor having the structure of FIG. 3 has been considered. The structure is such that in order to reduce the inductance, permanent magnets 13 and 14 to be provided in the series magnetic circuit of the windings are mounted on the surfaces of poles, and in order to enlarge the change of flux interlinkage with the windings 3, 4, 5 and 6, the polarity of poles of the magnets is inverted corresponding to the pitch of the teeth of the poles, so that the reduction of thrust is prevented. In the structure, however, a magnetic leakage path is formed between adjacent poles of the permanent magnets to reduce effective flux in the gaps, so that the flux interlinkage with the windings is reduced. Accordingly, sufficient thrust cannot be attained even by this structure. In order to reduce the flux leakage, the permanent magnets must be thinned sufficiently relative to the pitch of the poles of the permanent magnets. However, the magnetic reluctance of the magnets decreases as the thickness thereof decreases. Accordingly, a problem arises in that the inductance of the windings increases. Further, in the case where the pitch of the poles of the permanent magnets are enlarged, the pitch of the teeth must be enlarged with the enlargement of the pitch of the poles of the permanent magnets. Accordingly, a problem arises in that the thrust constant is lowered because the change of flux relative to the positional change of the movable member is reduced.
Consequently, even in the case where the polarity of poles of the magnets are inverted corresponding to the pitch of the teeth in order to enlarge the change of flux, it has been difficult to make the reduction of inductance compatible with the improvement of thrust constant.
An example of the conventional stepping motor in which resistance is increased to thereby improve response is disclosed in Takashi Kenjo and et al., "Principle and Application of Stepping Motor", Sogo Electronics Publishing Co., Ltd., pages 180 to 182, February 1979, an example of the conventional stepping motor in which a magnet is provided in the magnetic circuit of the windings is disclosed in Japanese Patent Unexamined Publication JP-A-60-200757, and an example of the conventional stepping motor in which the polarity of poles of a permanent magnet is inverted at a pitch equal to the pitch of teeth is disclosed in Osahiko Nagasaka, "Study of Prototype PM Linear Pulse Motor", Magnetics Research of the Institute of Electrical Engineers of Japan, MAG-85-130, 1985.
Consequently, the aforementioned first conventional technique has the problem that the thrust constant is lowered with the attempt to reduce the electrical time constant, because the technique is not under sufficient consideration as to the relation between the thrust constant and the electrical time constant in the hybrid stepping actuator.
In the following, a motor driving method according to a second conventional technique will be described with reference to FIGS. 10A and 10B. In the drawings, the reference numeral 101 designates a section of a motor, and the reference numeral 102 designates a circuit for driving the motor. The motor has a pair of stators 103, and a movable member 108. The stators 103 are provided with A-phase and B-phase windings 104 and 106, respectively. The movable member 108 is provided with permanent magnets 109 disposed at the opposite sides thereof. Each of the permanent magnets 109 has a plurality of poles disposed at equal intervals at the pitch equal to the pitch .lambda. of the teeth of the stators 103 so that the poles N and S of each of the permanent magnets 109 alternate. The permanent magnets 109 are disposed in a manner so that one permanent magnet 109 being in opposition to the A-phase stator is shifted in phase by an electric angle of 90 degrees (1/4 of the teeth pitch .lambda.) relative to the other permanent magnet 109 being in opposition to the B-phase stator. The drive circuit 102 is provided with two groups of transistors 110, 111, 120 and 121, and 112, 113, 122 and 123. The one group of transistors 110, 111, 120 and 121 are connected to each other in the form of H through the A-phase winding 104, while the other group of transistors 112, 113, 122 and 123 are connected to each other in the form of H through the B-phase winding 106, the one and the other transistor group being connected across a DC source 131, as shown in FIG. 10B. The transistors are ON-OFF controlled by a control circuit 130 which is supplied with a signal from a sensor circuit 132 for sensing the position of the movable member 108.
In the motor, the direction of flux interlinked with the windings 104 and 106 is inverted in accordance with the position of the movable member 108 so that the direction of a current flowing in the windings 104 and 106 is inverted corresponding to the change of flux to thereby generate thrust in the movable member. The changes of flux interlinkage with the respective windings 104 and 106 are shifted in phase by 90 degrees from each other corresponding to the shifting of the position of the permanent magnets 108 and 109. Accordingly, if the currents of the A-phase windings 104 and B-phase windings 106 are inverted in accordance with the respective phases of the currents, unidirectional thrust can be generated in the movable member continuously at any position thereof. The groups of H-connected transistors in the driving circuit 102 are driven to operate in a manner as follows. That is, in the one group of H-connected transistors associated with the A-phase winding 104, the transistors 110 and 121 are turn on at a certain point of time to pass a current from the terminal a to the terminal b, whereafter the transistors 111 and 120 are turn on at the next point of time to pass a current from the terminal b to the terminal a reversely. Also the other group of H-connected transistors associated with the B-phase winding 106 are driven to operate in the same manner as the above-mentioned one group of transistors. The aforementioned reversible operation of current flowing in the windings 104 and 106 can be accomplished by the foregoing ON-OFF operation of the transistors. The transistors are controlled by control signals from the control circuit 130. The control circuit 130 receives the position signal from the sensor circuit 132 for sensing the position of the movable member 108 so as to judge whether the current is to be inverted or not to thereby control the current. Alternatively, the control circuit 130 may judge the inversion of the current by itself to thereby control the current by so-called open-loop control without using the position signal.
The motor of this type has an advantage in that only thrust can be enlarged by minimizing the pitch of the teeth with the motor rating kept fixed, because the magnitude of thrust is in proportion to the change of flux interlinkage with the windings. Accordingly, a load can be moved at a high speed by use of such a motor driving method. Accordingly, the motor of this type can be suitably used as an actuator for feeding a head in a disk drive or the like.
An example of the motor of this type is disclosed in Japanese Patent Unexamined Publication JP-A-56-74080.
The aforementioned second conventional technique has an advantage in that large thrust driving in a motor can be attained, but has a problem in that the current flowing in the windings is reduced with the increase of reactance and with the rising of the induced voltage when the motor becomes into a high-speed running condition, because the technique is not under sufficient consideration as to the maintenance of the large thrust in the high-speed condition.
In the following, a field magnet used in stepping motors and linear pulse motors according to a third conventional technique, will be described.
Generally, a linear pulse motor using a field magnet is formed as described in Japanese Patent Unexamined Publication JP-A-56-74080. According to the above Japanese Patent Unexamined Publication, a stator is constituted by an elongated base and a plurality of permanent magnets fixed to the upper surface of the base. The permanent magnets are magnetized to provide N and S poles alternately in a direction of movement of an armature along the stator.
As described above, since the poles of the field magnet in the conventional motor are formed by magnetization at a fine pitch, so that large magnetomotive force cannot be generated.
Accordingly, the thrust of the conventional motor is too small to deal with a large load. This causes limitation in the purposes of use of the motors or actuators of this type.